Why does an aft CG result in lighter than normal control forces?

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Revtach

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Revtach
I'm trying to understand a concept in the PHAK. Chapter 10 (weight and balance) states that:

"Tail heavy loading also produces very light control forces, another undesirable characteristic. This makes it easy for the pilot to inadvertently overstress an aircraft."

The "Tail heavy loading" above is meant in reference to loading with an aft center of gravity and the effects of stability. The book does not further explain how an aft CG causes light control forces and I'm trying to understand how. The only explanation I've been able to think of would be that perhaps the aft CG also means that the arm from the CG to the tail is shorter, and therefore your tail forces have less leverage to rotate the airplane's nose up and down.

Can anyone explain this further?
 
An aft CG reduces the airplane's pitch stability, and reduces the required tail downforce needed to maintain level flight. With reduced pitch stability it takes less elevator deflection to produce a pitch change.
 
Stability in aircraft design requires downforce at the tail. Aft CG means the downforce necessary is reduced, which means stability is reduced.

Edit: @Dana beat me to it!
 
As the CG moves aft and gets closer to the airplane's neutral point the restoring moment due to disturbance away from trim decreases - this is what you think of as reduced stability. The resulting angular acceleration is due to net moment, or the moment caused by the control surface minus the restoring moment of natural stability. Since the restoring moment is decreasing but the moment due to the control surface is not (or not as much), the resulting angular acceleration is larger. In the pitch axis this translates to a larger change in AOA with aft CG than with forward for the same control input, which relates to the 'inadvertent overstress' of the OP.

Nauga,
momentarily.
 
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The center of lift is behind the center of gravity. The CG acts as a pivot point, and the CL being behind it raises the tail. The horizontal stabilizer is an airfoil that pulls itself downward - opposite to the wing - balancing the forces. The closer the CG is to the CL, the less downforce needed from the tail to keep the plane level; at aft CG the elevator has to be deflected less.

And since drag increases with lift (lift in any direction), when less downforce is produced there is less drag from the horizontal stabilizer and the plane is faster for the same thrust.
 
Revtach said:
I'm trying to understand a concept in the PHAK. Chapter 10 (weight and balance) states that:

"Tail heavy loading also produces very light control forces, another undesirable characteristic. This makes it easy for the pilot to inadvertently overstress an aircraft."

The "Tail heavy loading" above is meant in reference to loading with an aft center of gravity and the effects of stability. The book does not further explain how an aft CG causes light control forces and I'm trying to understand how. The only explanation I've been able to think of would be that perhaps the aft CG also means that the arm from the CG to the tail is shorter, and therefore your tail forces have less leverage to rotate the airplane's nose up and down.

Can anyone explain this further?
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Au contraire…

You need to go back to chapter 6 to understand the flight surfaces and controls.
 
Revtach said:
I'm trying to understand a concept in the PHAK. Chapter 10 (weight and balance) states that:

"Tail heavy loading also produces very light control forces, another undesirable characteristic. This makes it easy for the pilot to inadvertently overstress an aircraft."

The "Tail heavy loading" above is meant in reference to loading with an aft center of gravity and the effects of stability. The book does not further explain how an aft CG causes light control forces and I'm trying to understand how. The only explanation I've been able to think of would be that perhaps the aft CG also means that the arm from the CG to the tail is shorter, and therefore your tail forces have less leverage to rotate the airplane's nose up and down.
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Try arm between CG and center of lift.
 
Daleandee said:
When a really fat kid is on the other end of the teeter totter, they are easier to lift when they move closer to you ... ;)
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Something like this, with aviation, think about a wrench and torque. Get a really long wrench like 3 feet long, try to loosen a difficult to loosen bolt at 1’ / 2’ / 3’ distances from the bolt. Note the differences.

Similar for understanding CG and weight distribution along a point too. Also similar for understanding about a critical engine in multi-engine aircraft (where the prop exerts its force relative to the CG).
 
My understanding - I could have something wrong, but this is what I remember from ground school long ago.

Start with what makes our aircraft stable - the aerodynamic design. We know that our wings produce lift. But if that was the only lift produced, we would be out of control because that's a single line. The stabilizer / stabilator also produces lift. Both surfaces push upward and because the tail is further away from the CG, the smaller surface with the longer arm still produces the same moment as the main wings.

When they are balanced, we fly in a known pitch attitude. If we use the yoke to move away from that attitude, the longer arm of the tail surface produces more moment to return to stable point and we get stable flight.

Knowing that, it becomes easier to understand why moving the CG aft makes the control forces on tail lighter. Reducing the length of the arm reduces the moment the tail needs to produce. This also explains why there is a CG envelope and the impacts of moving outside the envelope.


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I noticed on the last flight I did I made some of the best landings I ever have. The only difference was we were loaded to about 50 lbs less than gross and landed with 20 gal less fuel than takeoff. I re-did the W&B and we were still well within the box, but further aft than my normal solo without much in the baggage area.
 
Think of an arrow - heavy up front, fletching in the back. The CG is well forward of the center or aerodynamic forces. Since it rotates around the CG, any deviation from straight lets the aero forces push the tail back in line. Hard to turn. The larger the separation (larger fletching) the more stable it is and the more effort it would take to turn it.
If the aero center and the CG are at the same place (a stick with no fletching or arrow head) it takes zero effort (control force) to make it rotate. There is no restoring moment to make it stable.
Then, try to fire an arrow backwards - with the aero center in front of the CG, what happens? Flips around by itself.

Another way to look at it - tricycle gear puts the CG in front of the main gear - keeps it stable takes a control force to make it turn. Tailwheel, the CG is behind the mains - likes to spin around all by itself. If the CG is at the main gear, it is neutrally stable - no effort required to make it turn, but it doesn't try to turn by itself.

Contrary to what Meghan Trainor says, it's all about that arm.
 
bflynn said:
My understanding - I could have something wrong, but this is what I remember from ground school long ago.

Start with what makes our aircraft stable - the aerodynamic design. We know that our wings produce lift. But if that was the only lift produced, we would be out of control because that's a single line. The stabilizer / stabilator also produces lift. Both surfaces push upward and because the tail is further away from the CG, the smaller surface with the longer arm still produces the same moment as the main wings.

When they are balanced, we fly in a known pitch attitude. If we use the yoke to move away from that attitude, the longer arm of the tail surface produces more moment to return to stable point and we get stable flight.

Knowing that, it becomes easier to understand why moving the CG aft makes the control forces on tail lighter. Reducing the length of the arm reduces the moment the tail needs to produce. This also explains why there is a CG envelope and the impacts of moving outside the envelope.


View attachment 123609
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Ugh.

Usually the tail is producing slightly downward pressure but the further aft the CG generally the less it needs to do so, up to the point where it becomes dangerously aft.

This is why an aft CG will allow you to go a tad faster but be harder to recover from a stall.

This is one reason why people try to design canards. Typically a canard is a lifting surface vs an elevator/ horizontal stabilizer not helping with lift that much.

Also, you CAN learn all this from the maligned-PHAK.
 
bflynn said:
The stabilizer / stabilator also produces lift. Both surfaces push upward and because the tail is further away from the CG, the smaller surface with the longer arm still produces the same moment as the main wings.

View attachment 123609
Click to expand...

If the tail actually did generate lift upwards, wouldn’t the plane just continuously flip tail over nose about the lateral axis since both are behind the CG?

Take that picture you had and change the lift arrow of the tail to point down. Then it would be correct. The downward “lift” of the tail offsets the heavy nose of the CG -usually- in front on the main wing center if lift. That’s what creates positive static stability. Move the CG too far back near the main center of lift and the plane becomes neutrally stable. It loses the tendency to fix itself when pitch is displaced from whatever it was trimmed for.


Sent from my iPhone using Tapatalk
 
Simple way to understand what’s going on: try to throw a dart tail first. Notice how it will tend to flip over and put the heavy point forward?

As the CG moves forward, the plane becomes more stable in pitch. Aftward makes it less stable. Too far forward and the plane becomes unresponsive to the flight controls; too far aft and the plane becomes unstable. Put the CG at an aft location where the plane is barely stable and it will be very twitchy, responding rapidly and excessively to a control movement.

The allowable CG range for your plane puts the plane into a region where the plane is acceptably controllable but not near instability.
 
Okay good. Now someone explain how Santa navigates by compass from the North Pole.

*Edit - does no one play with Guillows balsa gliders as a kid anymore?
 
bflynn said:
My understanding - I could have something wrong... The stabilizer / stabilator also produces lift. Both surfaces push upward and because the tail is further away from the CG, the smaller surface with the longer arm still produces the same moment as the main wings.



View attachment 123609
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Ah, no. And this is a really bad illustration. Yes, all the vectors are shown in the positive direction, but in reality the tail lift and wing pitching moment are both negative.

On a conventional design, the tail produces downforce, not lift as shown. And the wing's pitching moment is nose down for a conventional airfoil producing lift. It is correct that the sum of the forces and moments about the CG must be zero, but that's not stability, just equilibrium.

For static stability, what is necessary is that dCm/dα (the change of the whole aircraft's pitching moment divided by the change in AOA) be negative, i.e. the more you pitch up, the more the plane wants to pitch down. This can be achieved with a lifting tail, but it's not common.
 
Sac Arrow said:
Now someone explain how Santa navigates by compass from the North Pole.
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The north magnetic pole is about 300 miles from the geographic north pole where Santa's workshop is. And, since he starts out west of the international date line and finishes just east of the line he can easily avoid the magnetic pole.

Duh.
 
Capt. Geoffrey Thorpe said:
The north magnetic pole is about 300 miles from the geographic north pole where Santa's workshop is. And, since he starts out west of the international date line and finishes just east of the line he can easily avoid the magnetic pole.

Duh.
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Good points, but I’m also pretty sure the reindeer have an amazing sense of direction. Besides, Santa mostly flies by pilotage with only a little ded reckoning.
 
Of course, none of this is true with a canard.
 
Capt. Geoffrey Thorpe said:
The north magnetic pole is about 300 miles from the geographic north pole where Santa's workshop is. And, since he starts out west of the international date line and finishes just east of the line he can easily avoid the magnetic pole.

Duh.
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Are you saying Santa's workshop is underwater?
 
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